(1) Field of the Invention
This invention relates to an aluminum based battery and more particularly to an aluminum based battery having a controlled concentration of alkaline electrolyte wherein discharge rate is low and the anode efficiency is high. Presently, a high power density primary battery based on aluminum and silver oxide alkaline half cells provide sufficient energy for vehicle propulsion. The major advantage of this electrochemical system is the extraordinary current densities, on the order of 1600 mA/cm, which are readily achieved. These high current densities are indicative of facile electron transfer in both the anodic and the cathodic redox couples. The high densities in the alkaline aluminum/silver oxide cathodic couple may be attributed to the anomalous solid phase mobility of Ag.sup.+. Unlike other cations, the silver cation travels rapidly not only through the liquid phase but also through the solid phase of its salts. Therefore, as AgO is reduced, Ag.sup.+ can continually travel to the electrode interface, preventing surface passivation and permitting continuous facile electron transfer.
(2) Description of the Prior Art
Prior to the present invention, it had been demonstrated that the aluminum-silver oxide battery was capable of operating with aluminum efficiencies above 90 percent during discharge at current densities in the range of 500 to above 1500 milliamps per square centimeter using sodium or potassium hydroxide electrolytes at above 70.degree. C. and at concentrations above 4 molar. However, under those same operating conditions, aluminum efficiencies drop to less than 50 percent when the current density was lowered into the range of 30 milliamps per square centimeter. Certain underwater vehicles require high energy density batteries capable of operating at very low to very high drain rates (30 to 1500 milliamps per square centimeter). The aluminum and silver oxide battery system would be a useful energy source for such vehicles only if its energy density at low rates could be improved. The corollary requirement was that it must operate with high aluminum efficiencies (low corrosion rates) at low current densities, otherwise corrosion of the aluminum produces excessive gas and reduces the energy density by using excessive electrolyte and aluminum.
Examples of aluminum based alkaline batteries are disclosed in U.S. Pat. Nos. 3,953,239; 4,107,406 and 4,150,204.
It would be desirable to provide an aluminum based battery capable of producing high voltages and operating at high efficiencies when functioning at low discharge rates.